Fix descending choices building for OrderingFilter

django_filters/filters.py

@@ -741,9 +741,15 @@ def build_choices(self, fields, labels):
             (param, labels.get(field, _(pretty_name(param))))
             for field, param in fields.items()
         ]
+
+        # Descending labels are more complicated:
+        # - Generated from parameter name
+        # - Generated from ascending label
+        # - Provided a descending label
+        asc = dict(ascending)
         descending = [
-            ('-%s' % param, labels.get('-%s' % param, self.descending_fmt % label))
-            for param, label in ascending
+            ('-%s' % param, labels.get('-%s' % field, self.descending_fmt % asc[param]))
+            for field, param in fields.items()
         ]
 
         # interleave the ascending and descending choices

tests/test_filters.py

@@ -1508,17 +1508,28 @@ def test_field_labels(self):
 
     def test_field_labels_descending(self):
         f = OrderingFilter(
-            fields=['username'],
+            fields=(('a', 'b'), ('c', 'd'), ('e', 'f')),
             field_labels={
-                'username': 'BLABLA',
-                '-username': 'XYZXYZ',
+                'c': 'BLABLA',
+                'e': 'BLABLA',
+                '-e': 'XYZXYZ',
             }
         )
 
         self.assertEqual(list(f.field.choices), [
             ('', '---------'),
-            ('username', 'BLABLA'),
-            ('-username', 'XYZXYZ'),
+
+            # Generates from prettified asc param name
+            ('b', 'B'),
+            ('-b', 'B (descending)'),
+
+            # Generates from provided asc label
+            ('d', 'BLABLA'),
+            ('-d', 'BLABLA (descending)'),
+
+            # Given desc label supersedes generated label
+            ('f', 'BLABLA'),
+            ('-f', 'XYZXYZ'),
         ])
 
     def test_normalize_fields(self):